An image forming apparatus having a plurality of image forming units is conventionally provided, in which toner images are sequentially transferred from the image forming units arranged along a paper conveyor belt to paper on the belt. In this type of image forming apparatus, the misalignment of images transferred from the respective image forming units (i.e., a color shift) can be prevented by the technique called Registration, and further image density errors in the toner images can be prevented by the technique called Image Density Correction.
The image forming apparatus includes an optical sensor having a light emitting section and a light receiving section, which is to be used by the above techniques. The light emitting section irradiates the belt with light, while the light receiving section receives the light reflected from the belt and outputs a light sensitive signal indicative of the amount of the received light. Marks to be used for Registration or Image Density Correction are formed on the belt using the image forming units. The positions or image densities of the marks are measured based on the light sensitive signal from the light receiving section, taking advantage of the difference in reflectivity (or in amount of reflected light) between the bare belt surface and mark-formed areas. The errors in positions or densities of images to be transferred are corrected based on the result of the above measurement.
However, the reflectivity of the bare belt surface may vary with long-term use, which can lead to accuracy deterioration in measurement of the positions and image densities of the marks. In view of this, it has been proposed that the level of the light sensitive signal when the light receiving section receives the light reflected from the belt surface be adjusted to a target level by a sensor adjustment process.
In the case that a leakage current is generated on the light receiving section due to manufacturing variations of the light receiving section and/or change in surrounding environment, the light sensitive signal outputted by the light receiving section may show the influence of the leakage current, and therefore indicate a certain level even when the reflected light is not applied to the light receiving section. In this case, the sensor adjustment process may fail to adequately achieve the above adjustment.
Thus, there is a need in the art to provide an image forming apparatus in which the adjustment of the sensitivity of a sensor can be adequately achieved while the influence of a leakage current on the adjustment is suppressed.